In currently employed optical communications networks, messages are transmitted via optical fibres whereas the switching of the messages at the network nodes takes place electrically as previously. The messages incoming in the form of optical signals are for this purpose converted by suitable converters into electric signals, switched electrically, and then re-converted into optical signals.
In future optical communications networks, the switching is also to take place optically. In this context one often refers to "transparent" optical communications networks, as the message is transmitted from transmitter to receiver continuously as a light signal. Optical switches are of particular significance in such communications networks. Their function is to switch-over incoming optical signals to one of a plurality of outputs. Optical switches are required to facilitate high switching speeds, to attenuate passing-through light to the least extent possible, and also to be cost-efficient and reliable.
A number of different concepts have been developed for optical switches. Thus for example EP-B1-0 494 768 has disclosed an optical switch wherein a plurality of optical waveguides intersect at right angles in one plane. The intersection points are provided with a groove extending diagonally to the optical waveguides. An oil can be robotically introduced into this groove and sucked out again. Depending upon whether oil is present in the groove or not, a light beam passes through the intersection point or is laterally deflected into the intersecting waveguide. In the case of this known switch, it is possible to form switching matrices with a very large surface area; however the disadvantage exists that the switching speeds are very low due to the required robotic movements.
Another concept for an optical switch is known from an article by N. Keil et al. entitled "Polymer Waveguide Optical Switch With .rarw.40 dB Polarisation Independent Crosstalk", Electronic Letters, Mar. 28, 1996, Vol. 32, No. 7, p. 655-657. The switch described therein is based on the principle of an optical directional coupler. In directional couplers two optical waveguides arranged in one plane extend at a short distance one beside another over a coupling length L. An essential feature of the proposed concept is that the optical waveguides consist of a polymer which exhibits a considerably more marked thermo-optical effect than glass for example. This means that even small temperature changes strongly affect the refractive index of the polymer. With the aid of heating electrodes arranged in the region of the coupling length, the temperature of the two optical waveguides, and thus their refractive index, can be separately controlled. In this way the effective coupling length can be set such that light guided in a waveguide either remains in this waveguide (straight position) or is coupled into the adjacent waveguide (intersecting position). Switches of a higher order (e.g. 4.times.4 switches) are obtained by cascading a plurality of such 2.times.2 switches. It is disadvantageous however that in the case of this switch light is guided only in polymer waveguides which, despite having favourable thermo-optical properties, relatively strongly attenuate light of the wavelengths normally used.
JP-1-200 233 A Abstract has disclosed an optical switch in which two different waveguide channels can be operated using one electrode. Here the channels are connected with an intersection.